Because of excellent mechanical and electrical characteristics, as well as high heat resistance, polyimides have been widely used in various fields as molding materials, composite materials, and electrical and electronic materials.
In the field of electrical and electronic materials, lead-free solder is becoming mainstream from an environmental point of view. Since lead-free solder has a higher reflow temperature than before, there is a demand for polyimides having higher heat resistance than before.
One known technique for increasing the glass transition temperature (Tg) of polyimide is a copolymerization method. For example, according to Japanese Unexamined Patent Application Publication No. 2003-212995 (Patent Document 1) and No. 2003-212996 (Patent Document 2), a diamine compound or tetracarboxylic acid dianhydride having a particular structure is copolymerized with a conventional diamine compound and/or tetracarboxylic acid to improve the physical properties, such as heat resistance and mechanical characteristics, of polyimide while maintaining the intrinsic properties of the polyimide. However, new monomers are sometimes not versatile materials because of the use of expensive raw materials or the difficulty with which the new monomers are synthesized.
Another known technique for increasing the Tg of polyimide is a method for introducing a functional group for thermal crosslinking into an end of polyimide, as disclosed in Japanese Unexamined Patent Application Publication No. 2006-291003 (Patent Document 3). However, this method involves altering the molar ratio between a diamine compound monomer, a tetracarboxylic acid dianhydride monomer, and an end group compound in a predetermined range. This inevitably results in a decrease in the molecular weight of polyimide, possibly affecting the physical properties of the polyimide.
For example, a polyimide having the following formula (A) is known as a representative polyimide.

An aromatic polyimide having the formula (A) is widely used particularly in an electronic material field. Films manufactured from this polyimide have high heat resistance and excellent mechanical characteristics.
Likewise, an aromatic polyimide having the following formula (B) is mostly used in the form of a film and is mainly used in the electronic material field.

In spite of its high heat resistance, the aromatic polyimide having the formula (A) or (B) is called non-thermoplastic polyimide and cannot be used in thermoplastic applications, for example, hot-melt adhesives and injection molding materials.
Thus, various thermoplastic polyimides have been developed. For example, a polyimide having the following formula (C) has a glass transition temperature of approximately 215° C.

Polyimides having still higher glass transition temperatures, such as polyimides having the following formulas (D) and (E), have been developed (for example, Patent Documents 4 to 7: U.S. Pat. No. 4,965,337 and Japanese Unexamined Patent Application Publication No. 03-068629, No. 05-320339, and No. 62-068817).

In particular, the polyimide having the formula (E) has a glass transition temperature of approximately 250° C. and has very high heat resistance. In order to further increase the glass transition temperature of this polyimide, for example, a copolyimide manufactured by the copolymerization with an aromatic diamine compound having the following formula (F) has been developed (for example, Patent Document 8: Japanese Unexamined Patent Application Publication No. 03-047837).

In this copolyimide, 10% by mole of a diamine component of the polyimide having the formula (E) can be substituted by the diamine having the formula (F) to increase the glass transition temperature by approximately 10° C. Although the aromatic diamine component having the formula (F) may be increased to further raise the glass transition temperature, such copolymerization impairs the intrinsic resin properties of the formula (E), producing a completely different resin.
Thus, a technique for increasing the glass transition temperature of polyimide while maintaining the intrinsic properties of the polyimide, that is, the intrinsic properties resulting from its molecular structure has not been found.
The polyimide having the formula (A) is widely used particularly in an electronic material field. Although films manufactured from this polyimide have high heat resistance and excellent mechanical characteristics, the films are yellowish brown and cannot be used as optical materials.
Likewise, the polyimide having the following formula (B) is mostly used in the form of a film and is mainly used in an electronic material field. Although this polyimide has high heat resistance and excellent mechanical characteristics, its films are reddish brown and cannot be used as optical materials.
Many studies have been conducted to solve the substantial coloring of such polyimides. For example, a technique for introducing a fluorine-containing group, such as a fluoro group or a trifluoromethyl group, into the main chain skeleton of polyimide has been developed. For example, the present inventors have disclosed various polyimides in Japanese Unexamined Patent Application Publication No. 05-178991 (Patent Document 9), No. 05-255501 (Patent Document 10), and No. 06-207015 (Patent Document 11). However, these polyimides require very expensive raw materials to introduce essential substituents. This constitutes a barrier to practical use.
At the same time, improvements of optical properties, such as colorless transparency, and other physical properties of polyimide have also been tried without introducing an expensive fluorine-containing group. For example, an alicyclic acid dianhydride or a diamine compound is used to develop a polyimide having improved optical properties, such as colorless transparency. As an example of using an alicyclic compound monomer as an acid dianhydride, Japanese Unexamined Patent Application Publication No. 2005-15629 (Patent Document 12) discloses a polyimide having a structure expressed by the following formula (G) derived from 1,2,4,5-cyclohexanetetracarboxylic acid dianhydride.

As an example of using an alicyclic diamine compound as a diamine compound, WO 02/10253 (Patent Document 13) discloses a polyimide having a structure expressed by the following formula (H) derived from diaminomethyl-bicyclo[2.2.1]heptane.

The polyimides having the formulae (G) and (H) have excellent colorless transparency, as well as their intrinsic high heat resistance. These polyimides are finding various applications, for example, display material members, including LCD, and flexible circuit materials. Flexible circuit materials have already been commercialized using conventional polyimides.
These applications generally require a processing temperature above 200° C. However, the polyimides having the formulae (G) and (H) have coefficients of linear expansion of approximately 50 ppm/K at approximately 200° C. and possibly cause problems during processing. For example, because of such a high coefficient of linear expansion, in a multilayer flexible substrate composed of such a polyimide and copper foil having a coefficient of linear expansion of 17 ppm/K, the circuit may have low precision or warping resulting from a difference in the coefficient of linear expansion.
In view of the situations described above, also for polyimides having excellent colorless transparency, there is a demand for a modifying method by which the coefficient of linear expansion of polyimide can be control to adapt the polyimide to various processing conditions without impairing a remarkable physical property of colorless transparency.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-212995
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2003-212996
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2006-291003
[Patent Document 4] U.S. Pat. No. 4,965,337
[Patent Document 5] Japanese Unexamined Patent Application Publication No. 03-068629
[Patent Document 6] Japanese Unexamined Patent Application Publication No. 05-320339
[Patent Document 7] Japanese Unexamined Patent Application Publication No. 62-068817
[Patent Document 8] Japanese Unexamined Patent Application Publication No. 03-047837
[Patent Document 9] Japanese Unexamined Patent Application Publication No. 05-178991
[Patent Document 10] Japanese Unexamined Patent Application Publication No. 05-255501
[Patent Document 11] Japanese Unexamined Patent Application Publication No. 06-207015
[Patent Document 12] Japanese Unexamined Patent Application Publication No. 2005-15629 [Patent Document 13] WO 02/10253